docs: restructure dev docs for macOS and Linux

[dakl]

Summary

• Move Nix flake instructions from Install section to Development section
• Split Development into macOS and Linux (Nix) subsections
• Add Development Platforms section to CLAUDE.md

Follow-up to #100.

Test plan

• Verify README renders correctly on GitHub

🤖 Generated with Claude Code

[greptile-apps]

Greptile Summary

This PR improves the documentation structure by moving Nix flake instructions from the Install section (intended for end users) to the Development section, and organizing development instructions by platform (macOS and Linux).

• Moved Nix flake setup from README Install section to Development section under Linux (Nix) subsection
• Split Development section in README into macOS and Linux (Nix) subsections for clarity
• Added Development Platforms section to CLAUDE.md documenting the two supported development environments
• Clarified that macOS development works with standard npm install, while Linux requires Nix flake for Electron dependencies

The changes improve documentation clarity without affecting any functionality.

Confidence Score: 5/5

• This PR is safe to merge with no risk - documentation-only changes that improve clarity
• Perfect score given this is purely a documentation restructuring with no code changes, no functional modifications, and the content is accurate and consistent across both files
• No files require special attention

Important Files Changed

Filename	Overview
CLAUDE.md	Added Development Platforms section documenting macOS and Linux (Nix) setup differences
README.md	Moved Nix instructions from Install to Development section and split dev docs by platform

_{Last reviewed commit: 5e2f1f0}